You could do that if you knew enough about how it works in detail. In practice they're just gonna make as big a blob as they can muster and apply RL on it or something, and nobody will be any the wiser as to what the thing's feeling.
Neuroscientists don't share your (and other people here like phys318u below) level of ignorance about how brains work.
I'm sure they know more about brains than I do but I'm also sure they cannot derive from an abstract description of a computational process whether that process implements suffering or self-awareness or whatever, which is what I think we'd need to do this human-neurons-as-lego-bricks type research safely.